A Keplerian finder is used as a finder for a high-end lens-shutter camera. Being arranged with a field frame or a reticle near the focal point of an object lens, the Keplerian finder, which is constructed with an object lens, which has a positive refractive power and an eyepiece, is capable of providing a clear observation of a field of view, a border of a field of view, and various display indicators. In the Keplerian finder, an entrance pupil exists inside the finder or at the object side of the finder. The Keplerian finder also provides an advantage that, even if it is used as a "zoom finder" for continuous change of magnification or as a wide angle finder, it does not require a large object lens. A zoom finder is particularly popular.
Keplerian optical systems, as disclosed in Japanese Laid-Open patent applications H3-233420 and H6-242377 are well known. In such an optical system, the object lens system is constructed, respectively from the object side, with a first lens group which has a negative refractive power, a second lens group which has a positive refractive power, and a third lens group which has a negative refractive power.
In recent years, attempts have been made to provide a smaller finder with larger magnification for a lens-shutter camera. However, the invention disclosed in Japanese Laid-Open patent application H3-233420 etc. proposes a finder in which each of the object lens groups is constructed with only one lens. This small finder corrects aberration excellently. However, it has a zoom ratio of only about 2.times., which is not large enough. A finder which has a zoom ratio of more than 2.times. is disclosed in Japanese Laid-Open patent application H6-242377. However, each of the lens groups in such a finder is made up of more than one lens. In particular, the second lens group is constructed with a plurality of lenses, which increases manufacturing cost.